(1) Field of the Invention
This invention relates to a process for preparing a polyamide having a high elongation and a high impact strength by alkali polymerization of an .omega.-lactam.
(2) Description of the Prior Art
The so-called alkali polymerization process for polymerizing an .omega.-lactam by the actions of an alkali catalyst and a co-catalyst is known. A polyamide obtained according to this process is excellent in mechanical strength characteristics such as tensile strength, flexural strength and initial tensile modulus and this polyamide is used as a machine part and an industrial material. This polyamide, however, is not advantageous in that both the elongation and impact strength are low and the polyamide is hard and brittle. Therefore, this polyamide cannot be used in fields where softness is required.
Several proposals have heretofore been made to improve the elongation and impact strength of polyamides obtained by alkali polymerization of .omega.-lactams.
For example, British Pat. No. 1,067,153 discloses a process in which an .omega.-lactam is polymerized by the actions of an alkali catalyst and a polyurethane co-catalyst having an isocyanate group at the molecule ends or on the side chains thereof, which is obtained by reaction between a polyol having hydroxyl groups at the molecule ends or on the side chains and a diisocyanate. However, the impact strength of a polyamide obtained according to this process is not so improved. Moreover, as is well-known, this polyurethane co-catalyst is poor in the thermal stability and storage stability, resulting in various troubles in carrying out this process on an industrial scale.
Japanese Examined Patent Publication No. 20,475/68 discloses a process for preparing a polyamide having improved water absorption and antistatic property, wherein an .omega.-lactam is subjected to alkali polymerization in the presence of an alkali catalyst and a product having isocyanate groups at the molecule ends or on the side chains, which is obtained by reacting a polymer having on the side chains functional groups capable of reacting with an isocyanate group, with a diisocyanate. In this prior art reference, an amino group is referred to as an example of the functional group capable of reacting with an isocyanate group. However, a polymer having an amino group is not specifically disclosed and there is not given any working example concerning this polymer.